Suppose I have a process that waits on UDP packets, the unified local
IPC that we're discussing, other unix sockets, and stdin. It's awfully
nice if the local IPC can be handled using the same select/poll
mechanism as all the other messaging.

So use UDP, you still haven't backed up your performance
claims. Experiment, set the SO_NO_CHECK socket option to
"1" and see if that makes a difference performance wise
for local clients.

I did provide numbers for UDP latency, which is more critical for my own
application since most messages fit within a single packet. I haven't
done UDP bandwidth testing--I need to check how lmbench did it for the
unix socket and do the same for UDP. Local TCP was far slower than unix
sockets though.

But if performance is "so important", then you shouldn't really be
shying away from the shared memory suggestion and nothing is going to
top that (it eliminates all the copies, using flat out AF_UNIX over
UDP only truly eliminates some header processing, nothing more, the
copies are still there with AF_UNIX).

Yes, I realize that the receiver still has to do a copy. With large
messages this could be an issue. With small messages, I had assumed
that the cost of a recv() wouldn't be that much worse than the cost of
the sender doing a kill() to alert the receiver that a message is
waiting. Maybe I was wrong.

It might be interesting to try a combination of sysV msg queue and
signals to see how it stacks up. Project for tonight.